Malaria parasites have a complex life cycle in which several stages, asexual and sexual, are involved in the vertebrate host and mosquito vector. Our interests in malaria research concentrate on the molecular events which may be responsible for the parasite's survival and differentiation into distinct stages, e.g. gametocytogenesis in the erythrocytes. Stage-specific antigens of asexual (merozoites) and sexual (gametocytes) blood stage parasites. We have prepared a lambda gtll genomic DNA library from 3D7 in the titer of 1x10 to the sixth power pfu/ug. Using a pfhsp9O DNA probe, we can clearly demonstrate a 3.6 kb message on the Northern blot and 9 clones from the genomic library (2xlO to the fifth power pfu). CDNA library from both merozites and gametocytes is under construction. The inserts will be used in subtractive hybridization to yield novel genes for each stage and our emphasis will be on the ones involved in the differentiation. Developmental regulation of ribosomal RNA. Two different genes encode small subunit rRNAs: A-gene transcripts are predominant in asexual blood stage parasites while C-gene transcripts are predominant in the mosquito stages. To study the switch from blood sexual stage to mosquito stages, total RNAs from P. falciparum gametocytes, zygotes and ookinetes were isolated and prepared as Northern blots. Using oligonucleotide probes from different regions of A and C genes, it was found that in P. falciparum the switch from A to C gene expression involves the control of rRNA processing with accumulation of C-gene precursors in gametocytes. As the C-gene precursor RRNA appears, the dominant A-gene RRNA begins to undergo a selective and limited degradation and eventually A-gene transcripts are replaced by mature C-gene transcripts in the mosquito.